The permanent electric dipole moment of gold chloride, AuCl

The [19.20]0⁺-X¹Σ⁺ (0,0) band system of gold chloride, AuCl, has been studied using optical Stark spectroscopy. The [19.20]0 state is analysed as a ³ electronic state, and the observed Stark shifts analysed to determine ground and excited electronic state permanent electric dipole moments, el. A considerably smaller _el of 0.32 ± 0.17 D for the [19.20]0⁺ (v = 0) state, in comparison to that of 3.69 ± 0.02D for the X¹Σ⁺ (v = 0) state is observed. The experimental assignment of the [19.20]0⁺ state to a component of the ³Π state has been corroborated by high-level quantum-chemical calculations using exact two-component theory for treating relativistic effects and the equation-of-motion coupled-cluster approach for describing the electronic excited state. A close inspection of the electronic wave functions for the ³Π states of gold monohalides reveals significant participation of excitations from the halogen valence p orbitals to the anti-bonding molecular orbitals mainly localised on the gold atom. This leads to a charge transfer from halogen to gold and is responsible for the dramatic reduction of dipole moment in the ³Π states in comparison to the ground states as observed in the Stark-shift analysis. It has been further demonstrated that this ligand to metal charge transfer increases along the F to I series and leads to predicted dipole moments in the ³Π states of AuBr and AuI that point towards the gold atoms, qualitatively different than might be anticipated.